This invention relates to magnetic refrigerants which can effect cooling owing to their magneto-caloric effects. They are useful for magnetic refrigeration.
Reflecting significant developments in the superconductivity technology in recent years, the use of superconductivity is now being contemplated in a wide variety of fields such as industrial electronics, information industry and medical equipment. In order to make use of superconductive techniques, it is indispensable to develop refrigerators which may be used to produce superlow temperature environments. The gas refrigeration method has been well known. However, this method has extremely low efficiency and requires large apparatus for its practice. As a new refrigeration method which may replace the conventional gas refrigeration method, intensive researches are now under way with respect to the magnetic refrigeration method making use of the magneto-caloric effects of magnetic materials ("Proceedings of ICEC", 9, 26-29, May, 1982) "Advances in cryogenic Engineering", 29, 581-587, 1984), the contents of which are hereby incorporated by reference. Describing in short, the magnetic refrigeration method makes use of the endothermic-exothermic reaction which takes place owing to a change in entropy between an aligned spin state of a magnetic material upon application of a magnetic field to the magnetic material and a random spin state of the magnetic material upon removal of the magnetic field. This magnetic material is called a magnetic refrigerant". The magnetic refrigeration method has such inherent merits that it permits the use of smaller apparatus and brings about higher efficiency. It is thus believed to be a promising refrigeration method. The efficiency of magnetic refrigeration varies considerably from one magnetic refrigerant to another. Hence, each magnetic refrigerant is required to have large entropy and good thermal conductivity.
Researches have been made on various magnetic refrigerants. As exemplary magnetic refrigerants for target refrigeration temperature range of 20.degree. K. or lower, may be mentioned single crystals of garnet-type oxides which contain one or more rare earth elements, typified by Gd.sub.3 Ga.sub.5 O.sub.12 (GGG) and Dy.sub.3 Al.sub.5 O.sub.12 (DAG) ("Proceedings of ICEC", 30-33, May 1982; etc.). Illustrative of those intended for the temperature range of 77.degree. K. to 15.degree. K. include Laves-type RAl.sub.2 intermetallic compounds in which R stands for a rare earth metal ("Abstracts of Lectures of November, 1984", The Society of Cryogenic Engineering).
These magnetic refrigerants are required to develop large entropy changes (.DELTA.S) in their respective refrigeration temperature ranges. Taking a magnetic refrigerant for the wide temperature range of from 77.degree. K. (the temperature of liquid nitrogen) to 15.degree. K. by way of example, the magnetic refrigerant is required to show a large entropy change in the wide temperature range while having the same crystalline structure there. In order to develop such a large entropy change, it is necessary for the magnetic refrigerant to have magnetic transition temperatures which vary successively from one to another.
In intermetallic compounds of rare earth metals which have the same crystalline structure, the magnetic transition temperature which vary successively from one to another is readily obtained by substitution of the rare earth elements and thus a large entropy changes in the vicinity of magnetic transition temperature can be obtained in a wide range of temperature (for example from 77.degree. K. to 15.degree. K.)
As such magnetic materials, the above-described Laves-type RAl.sub.2 intermetallic compounds may be mentioned.
When the practical utility of a magnetic refrigerant is taken into consideration, the magnetic refrigerant is required to have high degrees of freedom and accuracy in machinability in addition to the above-mentioned characteristics. It will obviously be very effective if a sintered body satisfying all the above characteristics is obtained.
The above-mentioned Laves-type RAl.sub.2 intermetallic compounds are expected to have poor sinterability because the melting points of RAl.sub.2 are all high, i.e., above 1500.degree. C., although no report has been published as to the sintering of the Laves-type RAl.sub.2 intermetallic compounds. In view of the need for their sintering at high temperatures above 1500 .degree. C., they are accompanied by an economical drawback. They involve another economical drawback due to inclusion of the component R in large amounts. The high R contents lead to another problem, namely, low thermal conductivity. As a matter of fact, no effective sintered magnetic body has yet been obtained as a magnetic refrigerant suitable for use in magnetic refrigeration.
In magnetic refrigeration for the temperature range of 77.degree. K. to 15.degree. K. or so, it is desirable to employ a regenerater-type cycle such as the Ericsson cycle because the lattice entropy serves as a dominant contributor. In such a regenerater-type refrigerator, it is indispensable to maintain a high degree of thermal conduction between its magnetic refrigerant and refrigerant material. This thermal conduction gives substantial influenece to the cooling efficiency. Here, it should be noted that solid refrigerant materials such as lead are only available for superlow temperatures below 77.degree. K. It is thus necessary to establish solid-to-solid contact between the magnetic refrigerant and its associated refrigerant material or to form a narrow gap such as He gas film between the refrigerant and refrigerant material and to effect their heat exchange through the narrow gap. Whichever method is used, high-accuracy machining such as mirror finishing and machining of complex configurations is required for both magnetic refrigerant and refrigerant materials ("Abstracts of Lectures of November, 1984", The society of Cryogenic Engineering). For such regenerater-type refrigerators, there has been a special demand for the development of magnetic refrigerants having good machinability.